I came here looking for an answer to this very issue.
I believe that the limitation only being on having to be adjacent to your ally, goes hand in hand with the limitation that you can only aid their defense.
I also believe that you can't help defend against a ranged attack, because your making a melee attack, and there is no way for a melee attack to deflect arrows unless you have the monk feats that allow for it. Also, given the ability to defend against range attacks is giving this feat something the original feat doesn't have.